Everton rejected a number of bids for the defender during the summer from United, which all came after manager David Moyes ended his 11-year stay at Goodison Park to take charge at Old Trafford.

Martinez, though, repeatedly stressed since arriving at the club in June that he was determined to keep hold of the player.

The Spaniard told his news conference ahead of Tuesday's Merseyside derby: "It would be an understatement to say we are delighted to secure the best footballing years of Leighton Baines at our club.

"It's a massive boost and exciting for the future. Leighton has just turned 29 and brings the right measure of maturity and football knowledge in a very specialised position on the pitch, bringing an infectious and positive influence to the rest of the squad.

"I know that every fan will be delighted with the news and we all share that excitement looking at what's left of this campaign and beyond. With the crucial games we have ahead, it is going to be vitally important to have Leighton's experience there to help us fight for our aims."

Martinez added: "To have Baines secure and spending the best years of his career at Everton means a lot -- especially in this January window, when we are talking about making sure we have a strong squad to take us into the second half of the season."

Everton had been in talks with Baines over a new deal since the last transfer window closed in early September.

His previous contract had been set to expire in June 2015, and Everton knew they would have been especially vulnerable to offers from other clubs had he entered the final year of that contract without signing a new one.

The extension to his deal is a significant boost for the club as they prepare for Tuesday's clash against Liverpool at Anfield.

Martinez added: "It's a long-term deal, so it's for the next four years. I know the fans will be delighted, because Leighton is more than a footballer at our club. He represents our strong intent to progress, to get stronger.

"Leighton will bring all of that on the pitch. For me, he's one of the best left-backs in European football. I think his best is still to come."

The full-back joined Everton from Wigan Athletic in July 2007 and has gone on to become one of the most consistent players in the Premier League.

Baines, who has won 22 England caps, made his international debut in 2010 and looks to have recently dislodged Ashley Cole in Roy Hodgson’s first XI.